Numbers 8:13-19

13Have the Levites stand in front of Aaron and his sons and then present them as a wave offering to the LORD .

14In this way you are to set the Levites apart from the other Israelites, and the Levites will be mine. 15“After you have purified the Levites and presented them as a wave offering, they are to come to do their work at the tent of meeting. 16They are the Israelites who are to be given wholly to me. I have taken them as my own in place of the firstborn, the first male offspring from every Israelite woman. 17Every firstborn male in Israel, whether human or animal, is mine. When I struck down all the firstborn in Egypt, I set them apart for myself. 18And I have taken the Levites in place of all the firstborn sons in Israel.

19From among all the Israelites, I have given the Levites as gifts to Aaron and his sons to do the work at the tent of meeting on behalf of the Israelites and to make atonement for them so that no plague will strike the Israelites when they go near the sanctuary.”